The present invention relates to an optical fiber, and more particularly to an optical fiber having a normalized frequency applicable to a broadband optical fiber device.
The present invention also relates to an optical fiber device, and more particularly to broadband. polarization-independent, low-loss optical fiber devices which are achieved by using the aforementioned optical fiber along with a mode stripper or a mode selective coupler.
It is well known that a two mode optical fiber device using a mode coupling is between fundamental mode LP01 and second mode LP11 can be applied to an optical frequency modulator or a wavelength-tunable filter. The wavelength band width of this two mode optical device is determined by the dispersion of the beat length of the two coupled modes. Up to the present, the two mode optical fiber device has been used as a filter having a characteristic of a relatively narrow band wavelength with its linewidth being generally same or less than 10 nm. However, the wavelength dependency of the beat length disappears at a wavelength in which the two space modes have a same group velocity. That is, a device using a coupling between these two modes can be operated in a broad wavelength band, since the dispersion of the beat length is small in the vicinity of this wavelength.
In respect to a circular core optical fiber having a step refractive index, the normalized frequency which is dimensionless value defined in a wave guide is represented by the following equation 1:                     V        =                                            2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              r                co                                      λ                    ⁢                                                    n                co                2                            -                              n                cl                2                                                                        (                  equation          ⁢                      xe2x80x83                    ⁢          1                )            
where, V is a normalized frequency, rco is a radius of the core, xcex is a light wavelength in vacuum, nco is a refractive index of the core, and ncl is a refractive index of the cladding, respectively. In the case of a circular core optical fiber having a step refractive index, the first normalized frequency has a value of about 3 in which the dispersion of the beat length between LP01 mode and LP11 mode reaches a minimum. In the case of high order modes which are higher than LP11 mode, the normalized frequency has a value which is same or more than 3 in which the dispersion of the beat length is the minimum depending on the mode. In turn, in a single mode optical fiber where coupling of LP01 mode propagating along the core and a cladding mode propagating along the cladding, the normalized frequency has a value which is same or less than 2.4 in which the dispersion is the minimum. For example, in the case of a common circular core optical fiber having a step refractive index, a normalized frequency with the dispersion of the beat length between LP01 mode of the core and LP11 mode of the cladding being the minimum has a value of about 0.7, and a normalized frequency with the dispersion of the beat length between LP01 mode and LP14 mode being the minimum has a value of about 1.3.
That is, the exact value of the normalized frequency, where the dispersion is the minimum, is determined depending on the structure of an optical fiber, i.e., a distribution of the refractive index. Therefore, a broadband optical fiber device can be achieved, by properly adjusting a core radius, a core refractive index and a cladding refractive index that are structural parameters of an optical fiber, to have a normalized frequency value in which the dispersion of the beat length between LP01 and LP11 modes are the minimum in a broad optical wavelength band.
Accordingly, it is an object of the present invention to provide an optical fiber having a normalized frequency value applicable to optical devices which operate in a broad wavelength band.
It is other object of the present invention to achieve various broadband, polarization-independent, low-loss optical fiber devices by using the aforementioned optical fibers.
To achieve the above object, there is provided an optical fiber having a normalized frequency value, in which the dispersion of the beat length between a first and a second modes is a minimum in an optical signal wavelength band, by adjusting structural parameters of the optical fiber including a core radius, a core refractive index and a cladding refractive index, so that the beat length between the first and second modes does not depend on an optical wavelength in the operating wavelength band of the passing light.
The optical fiber is preferably selected by an elliptical core two mode optical fiber which has an ellipticity same or less than 0.9. The ellipticity is referred to a ratio of the minor axis to the major axis in this case.
Otherwise, the optical fiber may preferably be selected as a single mode optical fiber so that the first and second modes correspond to the core and cladding modes, respectively.
The optical fiber device of the present invention further comprises a single mode optical fiber for introducing incident light into the above-described structural parameter adjusted optical fiber; acoustic wave generating means which generates a flexural acoustic wave in the structural parameter adjusted optical fiber; and output mode stripping/selecting means, connected to the optical fiber with structural parameters adjusted, for eliminating or selecting a particular mode of the light that passes therethrough.
In this case, the structural parameter adjusted optical fiber may also use an elliptical core two mode optical fiber which has an ellipticity same or less than 0.9, or a single mode optical fiber. In case of using the single mode optical fiber, the first and second modes correspond to the core and cladding modes, respectively.
In constituting the optical fiber device, input mode stripping/selecting means may be further provided between the single mode optical fiber and the structural parameter adjusted optical fiber.
Alternatively, as the input mode selecting means, a mode stripper may be used which is an optical fiber wound with a small diameter to remove high order modes with bending losses.
The input mode selecting means can be a mode selective coupler made with the structural parameter adjusted optical fiber and another single mode optical fiber which has the same effective refractive index as the refractive index of the second mode of the structural parameter adjusted optical fiber, so that the optical fiber device can be operated as a 1xc3x972 optical switch.
As the input mode selecting means, a mode stripper may be used which is an optical fiber where the single mode optical fiber and the structural parameter adjusted optical fiber spliced together and tapered at the spliced portion, to allow only one mode to pass.
The output mode stripping selecting means for the passing light may be comprised of a mode selective coupler made with a single mode optical fiber which have same effective refractive index as the refractive index of the second mode in the structural parameter adjusted optical fiber, so that the optical fiber device can be operated as a 1xc3x972 optical switch.
The output mode stripping/selecting means for the passing light may be comprised of a mode stripper which is an optical fiber wound with a small diameter to strip high order modes with bending loss. or a mode stripper which is a tapered optical fiber to allow only one mode to pass. so that the optical fiber device can be operated as a 1xc3x971 optical switch.
If the mode stripping/selecting means for the passing light is comprised of a mode selective coupler, the optical fiber device of the present invention can perform a function of a 2xc3x972 optical switch or an add/drop multiplexer.
Furthermore, the single mode introducing optical fiber and the structural parameter adjusted optical fiber are preferably inserted into a tube, the material of which has a similar thermal expansion coefficient as the optical fibers, in order to prevent length variation of the optical fibers depending on temperature variation and to protect the optical fibers.
In addition, the above-described several optical devices may be used in connection to perform a function of a matrix switch, or otherwise only one may be used to perform a function of a loop-back switch which sends back optical signal inputs.